Our work concerns the structure and function of certain genes in the fruit fly, Drosophila melanogaster, and divides into two general areas. First, by genetic and physical studies we intend to understand the mechanisms controlling increases or decreases in the copy number of redundant gene loci such as the ribosomal RNA genes (rDNA) and 5S RNA genes (5S DNA). We have now identified two genes that control rDNA redundancy. The second area of interest concerns the isolation and characterization of chromosome pairing sites that permit homologous chromosomes to recognize each other and pair before undergoing gene recombination. In this regard we have isolated a clone within one band of a pairing site and are "walking" into this locus.